Gold has been prized across history for its unique luster, resistance to corrosion, and inherent scarcity. It has served as a primary store of value for civilizations worldwide for thousands of years. While gold is naturally occurring, it is almost never found in a chemically pure state and requires refining before it is suitable for coinage, jewelry, or industrial use. Gold purification involves intense thermal processing, where the application of extreme heat, or “fire,” is the central mechanism for separating the desirable metal from unwanted contaminants.
Understanding Crude Gold and Impurities
The raw material derived from mining and initial smelting is often referred to as dore metal or crude gold. This alloy is typically less than 90% pure, containing a significant proportion of other elements. The most common impurity is silver, which naturally alloys with gold in the Earth’s crust.
Other base metals frequently found in the dore include copper, iron, zinc, and lead. These contaminants negatively affect gold’s color, malleability, and market value. Refining exploits the differing chemical properties and melting points between gold and these other metals, allowing for the selective removal of impurities under controlled conditions.
The Fundamental Role of Heat and Fluxing Agents
The initial purification relies on the application of high temperatures, often exceeding gold’s melting point of 1,064 degrees Celsius (1,943 degrees Fahrenheit). Traditional fire refining methods, such as smelting, use this heat alongside specialized chemical additives known as fluxing agents. Fluxes are materials like borax, soda ash, and silica, which chemically react with non-gold impurities, particularly those that have oxidized.
These reactions create a low-density liquid waste product called slag. The slag, being much lighter than the molten gold, floats to the surface of the crucible. This physical separation allows the waste material to be skimmed off, leaving behind a more concentrated gold alloy. A related technique is cupellation, which uses a porous material called a cupel to absorb oxidized base metals, such as lead oxide, under high heat, leaving a bead of precious metal behind.
Modern Chemical Refining Using High Temperatures (The Miller Process)
While traditional fluxing methods offer a good degree of purification, modern refining demands higher purity levels, leading to the development of processes like the Miller Process. Patented in 1867, this chemical refining technique uses intense heat combined with a gaseous reagent. The crude gold is melted in a furnace at around 1,000 degrees Celsius, keeping the metal in a liquid state.
Chlorine gas is systematically pumped through the molten gold bath, taking advantage of selective chemical affinity. At this high temperature, the chlorine gas readily reacts with nearly all base metal impurities, such as zinc and iron, to form metal chlorides. These chlorides are volatile compounds that escape the furnace as a gas, physically separating themselves from the gold.
As chlorine continues to be introduced, it next reacts with silver and copper, the most significant remaining impurities. The resulting silver and copper chlorides form a molten slag layer that floats on the surface of the denser, purer gold. This slag is then physically removed or skimmed from the top of the melt. Gold itself remains largely unaffected by the chlorine at this temperature because gold chloride is unstable above approximately 400 degrees Celsius, preventing the gold from reacting and escaping.
Final Purity Standards and Measurement
Following the thermal refining process, the purity of the resulting gold is determined using two primary measurement systems. Fineness expresses the gold content in parts per thousand, with 999.9 fineness representing 99.99% pure gold. The second system, the karat scale, uses 24-karat (24K) as the benchmark for pure gold.
The Miller Process is highly efficient and typically yields gold with a fineness of approximately 995 (99.5% pure). This level of purity is sufficient for many industrial applications and is the standard for certain gold bullion products. To achieve the highest investment-grade purities, such as the 999.9 fineness required for “four nines” gold, the Miller Process product often requires a subsequent purification step, such as the non-fire Wohlwill electrolytic process.